1. Field of the Invention
The present invention relates to a capillary used for wire bonding and to a method of wire bonding using such a capillary. More particularly, the present invention relates to a capillary used to mount semiconductor devices and the like, and to a method of wire bonding using such a capillary.
2. Description of the Prior Art
A conventionally widely practiced method of mounting a semiconductor device on a leadframe or circuit board is wire bonding, a method whereby electrodes are connected together with wires. Common wire bonding is performed roughly in the following manner. First, the tip of wire extracted from a capillary through the opening formed at the tip thereof is formed into a ball-like shape by electric discharge or the like. Then, the capillary is moved to above one electrode and then down so that the tip of the wire formed into the ball-like shape bonds to the electrode (this operation is called first bonding). At this time, to increase the bonding strength with which the wire bonds, a supersonic vibration is applied thereto. Next, the capillary is moved to above another electrode and then down so that the wire bonds to the electrode (this operation is called second bonding). Also at this time, to increase the bonding strength with which the wire bonds, a supersonic vibration is applied thereto.
Here, the second bonding is achieved by pressing and thereby deforming the wire with the face surface of the capillary so that the wire bonds to the electrode, a method called stitch bonding. In such stitch bonding, the shape of the face surface of the capillary, i.e., the surface at the tip end thereof, greatly influences how the wire bonds to the electrode.
As shown in FIG. 4, the shape at and around the tip of a conventional capillary 1′ is such that, relative to the thickness of wire 2, the width of the capillary (if it is cylindrical, its diameter) is small, and the area of its face surface 13 is also small. For this reason, the curved-surface portion 15 of the capillary, i.e., the portion thereof rounded to connect the face surface 13 to the peripheral surface 14 thereof, is also used to press and deform the wire 2.
However, with the curved-surface portion 15 of the capillary, the wire 2 cannot be pressed and deformed to a sufficient degree. This sometimes leaves part of the wire 2 unbonded and suspended off the electrode 31. In the fabrication process, such a gap between the wire 2 and the electrode 31 causes mechanical and thermal stress to concentrate at the boundary between the bonded and unbonded portions of the wire 2. This sometimes causes the wire to break at that portion.
This phenomenon occurs relatively frequently when second bonding is performed on top-surface electrodes of semiconductor devices. Through experiments, the inventor of the present invention has found that such wire breakage occurs with a far higher probability, namely in one in several tens of top-surface electrodes, when second bonding is performed thereon than when first bonding is performed thereon, in which case the probability is in one in several tens of thousands of top-surface electrodes.